Traction of a vehicle is established as its wheels contact a surface so that when the wheels are rotated, usually by a driving force, the vehicle will be moved along the surface in a desired direction. The combination of the coefficient of friction, mu (μ), and the force exerted by a wheel against the surface produces traction. When the coefficient of friction of the surface is less than the force exerted, the wheel will slip during acceleration of the vehicle, adversely affecting acceleration performance and driving stability. Slippage can occur as a result of excessive accelerative forces applied to vehicle wheels or inadequate wheel to surface friction that can be present with wet or icy conditions, as well as in other surface conditions. Once the condition is recognized, a vehicle driver, particularly in an automobile or like vehicle, may try to control slippage by reducing engine power or by applying the brakes, both of which can reduce the speed at which a drive wheel is rotating. The driver may not be aware that slippage is occurring, however, and may not be able to take corrective action as quickly as required. Traction should be controlled in a manner that does not need driver action so that, at a minimum, slipping may be limited or, optimally, a vehicle's wheels may be prevented from slipping during acceleration on different surfaces.
Aircraft are required to travel between landing and takeoff on ground surfaces that may vary in quality, and traction control during taxi can be a challenge, particularly under adverse weather or runway conditions. Aircraft ground travel is presently conducted by using thrust from the aircraft's main engines and/or by tow vehicles to move aircraft between runways and gates or parking locations after landing and prior to takeoff. Apart from the application of the aircraft's brakes, simple effective traction control has not heretofore been possible during this type of aircraft ground movement.
Antiskid and traction control systems have been proposed for vehicles that travel on ground surfaces, including aircraft. Such systems have typically been used in connection with a vehicle's brakes. Most available systems include an antiskid controller that compares the vehicle speed derived from a wheel speed sensor to the vehicle speed derived from another source. If the wheel is determined to be skidding or slipping an excessive amount, the vehicle brakes are released, and the wheel is permitted to spin at an appropriate speed to move the vehicle on the surface. The determination of what amount of slipping is excessive and what amount of slipping is appropriate presents challenges for antiskid system brake controllers. Some vehicle antiskid control systems provide a model of a mu-slip curve that describes tire-to-road surface friction characteristics. Slip velocity, the difference between wheel velocity and vehicle velocity, is compared with a predetermined set point on the mu-slip curve to enable the controller to produce an appropriate amount of slip during vehicle travel. For aircraft, an antiskid controller should prevent excessive slipping or skidding when the aircraft lands and while the aircraft is taxiing between landing and takeoff.
A tire mu-slip curve is usually used to describe the developed friction of a braked tire and compares slip ratio with tire friction. A slip ratio of zero represents a free rolling tire and a slip ratio of 1.0 represents a locked wheel. The slip ratio and developed friction increase as braking is increased and maximum friction is reached. Ideally, an antiskid system operates at the maximum friction point. Operation beyond the maximum friction point produces skids or slips, while a decrease in friction without a corresponding decrease in braking will produce a locked wheel. When a tire is slipping slightly, it has more traction than when it is not slipping. Unlike the driver of an automobile or similar vehicle, an aircraft pilot is not likely to hear or feel a locking wheel. Without a functioning antiskid control system, a locked aircraft wheel can blow a tire in as little as 3 milliseconds at high speeds.
The peak of the mu-slip curve representing maximum friction and the location of the peak, although factors affecting the location are not fully understood, have been important considerations in the design of available vehicle antiskid control systems. Maximum friction generally occurs at about 10 to 20% slip. Operation of vehicle brakes at the peak of the mu-slip curve tends to produce the highest efficiency in current antiskid systems.
As noted, aircraft ground movement presently requires operation of the aircraft's main engines and assistance from tow vehicles. Controlling the speed of aircraft ground movement requires a combination of applying the aircraft's brakes to slow aircraft and increasing engine thrust to speed up the aircraft. Aircraft antiskid or traction control systems, as described above, may be used to modulate braking action to keep the aircraft moving under a range of ground surface and other conditions without skidding or locking wheels. Such systems are described in, for example, U.S. Pat. No. 5,135,290 to Cao; U.S. Pat. No. 5,918,951 to Rudd, III; and U.S. Pat. No. 6,125,318 to Zierolf. None of the aforementioned systems, however, suggests producing optimal traction in an aircraft with a landing gear drive wheel powered for ground movement by a non-engine drive means that is not dependent on friction values defined by a mu-slip curve.
Moving an aircraft on the ground without the operation of the aircraft's engines or the use of tow vehicles has been proposed. For example, U.S. Pat. No. 7,891,609 to Cox et al, owned in common with the present application, describes moving an aircraft along taxiways using at least one self propelled undercarriage wheel. McCoskey et al describes a powered nose aircraft wheel system useful in a method of taxiing an aircraft that can minimize the assistance needed from tugs and the aircraft engines in U.S. Pat. No. 7,445,178. Neither antiskid control or traction control nor a system that maximizes traction in an aircraft drive wheel during ground travel is suggested in either of these patents, however.
A need exists for a way to maximize traction in an aircraft drive wheel powered by non-engine drive means controllable to move the aircraft on the ground without reliance on the aircraft's main engines that produces optimal traction without dependence on the aircraft's brakes and friction values defined by a mu-slip curve.